1. Field of the Invention
The present invention relates to radio communication security, and more particularly, to digital radio apparatus and methods in which radio frequency hopping is used to provide security against interception of communications.
2. Description of the Prior Art
In the prior art, radio communication security has been provided by a technique called radio frequency hopping. In this scheme, as applied to digital radios, the transmitted radio frequency (RF) signal is typically encrypted to provide a first level of communication security. Further, the RF signal is radiated at different carrier RFs which are time sequenced, usually at a fixed hopping rate in accordance with a predefined pattern known to the intended RF receiver but not known to possible intercept receivers, thereby providing another level of communication security.
In particular, various RF communication systems, including SINGCARS, HAVEQUICK versions, JTIDS, etc., have employed frequency hopping or direct sequence spreading to achieve a spread RF spectrum at the tunable last transmitter stage and thereby to prevent intercept receivers from receiving transmitted information. These systems operate on the premise that a receiver must know the proper spreading pattern to receive the transmitted signal.
RF signal hopping provides communication security in the sense that an intercept receiver may detect a portion of a communication at a particular RF but cannot detect a whole communication which is transmitted by a series of hopped RFs.
With the hopping set of a transmitter not being known to an interceptor, a wide-band receiver can be used as an interceptor receiver to cover the entire spectrum over which the transmitted signal is hopped, but sensitivity is dramatically reduced (perhaps by 40 to 50 dB below a narrow band receiver). Additionally, the wide-band receiver will receive extraneous signals making it difficult or impossible to distinguish the target signal to be intercepted from the extraneous signals.
In any case, communication security may be seriously breached in the prior art through interception of transmitted leakage intermediate frequency (IF) or modulated baseband signals. Generally, transmitter RF circuitry is provided with filtering and conditioning functions which minimize these leakage signals, and the leakage signals are thus typically weak. However, the leakage signals are fixed in frequency and often have sufficient strength to be detected, especially by narrow-band receivers and especially over relatively short transmission distances.
In fact, where a potential intercept receiver is relatively close to the transmitter and the intended receiver is relatively distant from the transmitter, a leakage IF signal received by the intercept receiver may have greater strength than the RF signal received by the more distant, intended receiver. A need has thus existed to achieve higher communication security in the face of this leakage problem.